Laser pulses of approximately 300 fs duration (300 x 10(-15) s) were applied for second harmonic generation (SHG) at Si electrodes, for the first time. The frequency doubling by reflection at surfaces of centro-symmetric materials like Si is generated only at the surface due to the breaking of the crystal symmetry. This method is particularly interesting because even buried solid state interfaces can be studied. SHG as a non-linear optical effect needs high illumination power densities. This has restricted its use mainly to metal electrodes. Our recent studies with ns laser pulses showed that even semiconductor electrodes can be probed with SHG if the duration of the light pulses is shorter than any electron transfer reaction. A further advantage of fs pulses is the fact that illumination fluences well below the damage threshold, but still with sufficient power density, can be chosen. In contrast, the use of ns pulses required an operation near the damage threshold. Si electrodes in contact with aqueous electrolytes are normally covered by an oxide layer of SiO2 with a transition layer of non-stoichiometric suboxide SiO(x) (1 < x < 2). The extension of this layer as well as its growth mechanism and structure are still under discussion. Surface SHG promises to provide direct insight into this buried interface since the SH signal is generated exactly there. An oxide covered Si(111) surface yields an azimuthal SHG dependence as expected from the (111)C3nu symmetry. Removal of the oxide causes a drop in SH intensity with a subsequent rise ascribable to changes of the static electric field in the transition layer. At an oxide covered Si(111) electrode the SH signal can be modulated by an externally applied potential.